Can I get a stripped lower receiver, register it as a pistol when I get it from my local FFL.
Put a 11" pistol upper on it and a M-4 collapsable stock?
Can a registered PISTOL have a stock on it?
Or would this be a short barreled rifle and illegal?
Yes to the pistol, no to the stock. A stock makes it an SBR. Buy the tax stamp and register an SBR, you'll love it.